Deadhead roses regularly, otherwise plants will set seed rather than make more flowers. Healthy stems only need the flower head removing, but weak spindly ones need cutting back hard, to encourage new growth. Prune to where stems are at least pencil thick, even if it means removing almost the entire shoot.
Always cut just above a healthy, full-sized leaf. That's where the hormones concentrate, so the plant is able to produce a new flowering shoot quickly.